The present invention relates, in part, to forming --i.e., the shaping of a metallic object by means of pressure and at least some heat. In general, press forming has an advantage over other ways of shaping metallic objects in that the grain pattern, and thus the strength, of the formed object may be rearranged in a direction appropriate to the anticipated stresses which are to be encountered by the finished object. A common method of press forming is known as forging.
In general there are two broad categories of forging--viz., cold forging and hot forging. The demarcation between cold and hot forging is delineated by the recrystallization temperature.
Cold forging is the plastic deformation of a metal at such temperature and rate that strain-hardening occurs. Cold forging crushes and disturbs the grain structure to such an extent that hardness is increased, and ductility decreased, in proportion to the amount of deformation. Cold forging is noted for providing smooth finished surfaces and considerable dimensional accuracy. These are desirable results, but it must be appreciated that as the process continues, sufficient internal stresses are imposed until at some point the metal will fracture. Hence, for complex shapes which require extensive displacement of the metal cold forging is not a viable option.
Hot forging, on the other hand, is the plastic deformation of metal at such a temperature and rate that strain-hardening does not occur. The lowest temperature for this process is the recrystallization temperature. Hot forging tends to break up large grain structure to produce a fine grain structure, with minimal porosity.
The particular means by which the deformation of the metal is accomplished provides a further classification of the forging processes. There are, for example, drop forging, press forging, upset forging and roll forging. The present invention is most closely related to a press forging operation, but one which provides uniquely improved upset forging results.
Press forging of metallic parts is accomplished by positioning a slug of preheated metal in a shaping die that is secured within a forging press. The steady pressure applied to the slug by the ram of the forging press forces the metal into the shape defined by the die. With the longitudinal axis of the slug being aligned with the longitudinal axis of the ram, movement of the metal along the axis of the slug is called "gathering," and movement of the metal radially of the direction along which the metal "gathers" is called "spreading," or "upsetting."
Over the years a number of parameters have been established to determine the propriety of press forming a particular configuration. For example, if the shape has a central shaft portion and a cylindrical disk portion that extends radially outwardly from the shaft portion, one rule is that the axial dimension of the disk portion should not be more than three times the diameter of the shaft portion. A second rule is that if the length of the shaft portion is greater than three times the diameter of the shaft portion, the diameter of the disk portion must not be more that one and one-half times the diameter of the shaft portion.
Of utmost significance to the present invention, however, is the fact that when utilizing a fixed die in a forging operation it has been fundamentally understood that any significant "spreading" must occur within fairly close proximity to either end of the slug, and when the forging operation is employed primarily to produce radial spreading, as when forming a head on a bolt, the process is categorized as upset forging.
In those situations where the slug must be upset at a significant distance from the end of the slug, a sliding die set has been thought to be required.
Sliding die sets generally include a stationary die which often constitutes the means by which to grip the slug, or work piece. An additional die is located in spaced relation to the fixed die, and the additional die is movable toward and away from the fixed die along a frame which serves as the die guide. The slug, which is gripped by the fixed die, passes through the sliding die so that as the ram pressure of the forging press applies pressure against the slug to upset the slug, the sliding die moves toward the fixed die to shape the material as it is upset from the slug. Such an arrangement has been found to work quite satisfactorily when the slug is to be upset at a considerable distance from the end of the slug.
It is also possible to provide a plurality of sliding dies in one die set, but when the slug is to be upset at three or more locations along the length thereof to form a plurality of disk-like projections which extend radially outwardly from the central shaft portion, too many problems have been encountered, or anticipated, for either press forging or upset forging to be considered as viable options by which to manufacture such shapes.
The problems heretofore encountered, or anticipated, when employing press forging or upset forging to fabricate such shapes can be readily understood when considering the manufacture of the commonly employed spool valve element.
A spool valve is a mechanical valve in which a uniquely configured valve element is reciprocated at predetermined increments axially within a machined valve chamber in order to effect selective communication between ports which open into the valve chamber at spaced locations along the axis of the spool valve chamber. The typical valve element of a spool valve has a central shaft portion with a plurality of lands which extend radially outwardly of the shaft portion at spaced intervals along the longitudinal axis of the shaft portion. The aforesaid valve element is received within the chamber for relative axial translation, and the radially outer surface of the lands effect a sealing engagement with the surface of the valve chamber. By selectively locating the ports in conjunction with selective axially spacing of the lands, the recesses between successive lands are utilized to effect selective communication between successive ports in response to the particular axial disposition of the valve element within the valve chamber, as is well known to the art.
Because of the plurality of radially extending, longitudinally spaced lands, press forging, or upset forging, of spool valve elements has heretofore been deemed inappropriate.
Spool valve elements normally have a plurality of sharp corners--i.e., most surfaces intersect at substantially right angles. Because of the sharp corners, it has heretofore been envisioned that the metal flowing into the lobes of a die employed to form the disk-like lands will tend to fold back on itself and form "coldshuts." In addition, when a plurality of the disk-like lands are to be formed it has been envisioned that as metal in the slug "gathers" to provide the metal required to "upset" into that lobe in which the land most remote from the forging ram is being formed, the metal in the slug would be forced to flow past the other, intermediate lobes into which the metal may already have begun to spread in order to complete the formation of the most remote land. The formation of each successive land has likewise been thought to require gathering flow past die lobes into which the metal had already begun to upset. Such flow would, at best, induce shear stresses at the juncture of the shaft and the land portions formed by the intermediate lobes. At worst, one or more of the lands being so formed in the intermediate lobes might be virtually severed from the central shaft portion.
For these reasons metallic spool valve elements have heretofore been machined from bar stock having a slightly greater outside diameter than required to finish the radially outermost periphery of the lands, or the valve element has been cast and then finish machined. If accurately cast, the part could be completed by a centerless grinder, but in either event the cost of fabricating a spool valve element has, of apparent necessity, been relatively expensive.
The desirability to minimize the production cost of spool valve elements can be readily appreciated when one considers that a considerable number of spool valves are effectively employed, for example, in conjunction with automatic vehicular transmissions. Providing a spool valve element with a plurality of successive lands, with the appropriate recesses therebetween, allows for simultaneous, or sequential, hydraulic actuation of the various clutch and band assemblies required to effect the drive selection in a planetary gear set. In fact, it is not uncommon to utilize as many as six, or eight, spool valves to effect drive selection for a vehicular transmission.